The present invention relates to dielectric formation in the manufacture of solid state electrolytic capacitors, i.e. tantalum capacitors, and more particularly relates to selectively controlling the thickness of a dielectric formed on portions of an anodized electrode.
Solid state electrolytic capacitors are well known in the art and are typically fabricated by forming a pellet of powdered metal (e.g. tantalum) about an anode rod, sintering the pellet to form a porous mass bonded to the rod, forming a dielectric coating through the interior of the mass and exposed portions of the rod, e.g. by an anodizing step in a phosphoric acid bath, forming a conductive counter-electrode or cathode by overcoating the dielectric via manganizing or conductive polymer dipping, and terminating as by forming graphite and silver layers over the exterior surface of the counterelectrode.
The step of anodizing is a crucial step in the formation of a dielectric layer for use in solid state electrolytic capacitors, such as aluminum and tantalum. When, for example, an anodization is performed on a sintered anode in an electrolyte solution containing, for example, phosphoric acid, the thickness of the dielectric formed is in direct proportion to the time and applied voltage. The dielectric layer formed will be uniform throughout the anode as long as all portions of the anode are fully wetted by the electrolyte used in the process. In other words, the above-described conventional anodization process is non-selective, i.e., provides no mechanism for varying a depth of the dielectric on various portions of and within the anode body.
The inner body of the anode is responsible for contributing most of the finished capacitor's capacitance value. Consequently, the thickness of dielectric interposed between the anode and counter electrode directly defines the resulting capacitance. As a thickness of dielectric increases, the capacitance decreases but with consequent increase in working voltage of the capacitor. An increased dielectric thickness on the anode lead and the external anode surface (as distinguished from the inner portion) of the anode body provides increased protection against stress-related and other mechanical damages generated in post-anodization processes, such as in a step of manganizing the dielectric-coated anode. A dilemma therefore exists utilizing conventional anodization methods, which are non-selective, whether anode lead and body surface will receive a thicker dielectric layer and, hence, greater mechanical and electrical integrity at the expense of decreased capacitance. It would thus be desirable to provide a means for forming an increased dielectric thickness on external portions of the capacitor without increasing the thickness of the interior capacitance forming dielectric.